1. Field of the Invention
The present invention relates a process and its system for generating plasma with feedback control on plasma density. It particularly relates to a process that uses a heterodyne millimeter wave interferometer as sensor to measure the plasma density in the process container and the plasma density that is needed in the plasma generating process, and then provides real-time information of the measurements to a digital control device which makes numerical calculations and then drives the RF power generator to change RF output power so as to enable the plasma density in the plasma fabricating process to be close to the expected plasma density.
2. Description of the Related Art
The developing trend of the semiconductor industry is towards low cost and technology of fabricating wafers with high transistor density. So, to maintain strict operating parameters in the fabricating process is an essential prerequisite for guaranteeing a high product passing rate. Owing to this trend, real-time feedback control is gradually added to the system in the design of semiconductor fabricating equipment in recent years. For example, the equipment that is produced in quantities at present: rapid thermal processing (RTP), and chemical mechanical polishing (CMP), both adopt real-time feedback control to maintain the fabricating quality in the production process.
In the process of fabricating semi conductor components, the plasma process is a very important step. It includes etching, plasma deposition, ashing, wafer cleaning, plasma ion implanting and so on. All these need a stable steady plasma source to complete the fabrication of the semiconductor components. However, owing to the growth and change of the polymer formed by the process on the inner surface of the process container, the plasma density becomes unsteady. Meanwhile the interfering factors of the outer surrounding, such as the variation of power, gas supply and pressure, and the drifting of the input power, will also have influence on plasma source. For example, from FIG. 1 we can see that plasma density is directly proportional to RF input power. When the RF input power in the process container is steady, but the pressure changes, the plasma density will increase along with the increase of pressure. From FIG. 2 we can see that the changes of RF input power and the pressure in the process container have direct influence on the quality of plasma source.
In the process of plasma etching, plasma density has a direct effect on the etching rate and the critical dimension. So if we maintain effectively the steadiness of the plasma density, we can steadily control the etching rate and the quality of critical dimension.
The majority of quantity production plasma process equipment adopt open loop operation mode to control operation parameters, such as air pressure, RF power, gas flow rate, temperature and so on. However, when an abnormal situation of the plasma source in the equipment occurs, it cannot be immediately diagnosed. This often causes waste of cost and delay of delivery. So at present, SPC (statistical process control) mode and neural network mode are commonly adopted in the industry. These modes feedback to revise operation parameters according to the relation of the operation parameters and the component parameters. In this way, they can give early warnings of an abnormal situation of the plasma source and overcome the shortcomings of the open loop operation mode. However, when an abnormal situation of plasma source happens when we are measuring component parameters, there are still wafers in the process of production in the same equipment. Therefore the waste of cost is unavoidable.
In recent years, many research institutes participate actively in the development of real-time control system of plasma process. Sarfaty et al. (Conf. Rec. 1996 IEEE Int. Conf. Plasma Science pp. 281, 1996) used single wavelength and double wavelength laser interference technology as sensors to measure the real-time change of the film on wafers during the process of etching and deposition, and to coordinate with a driving device to form a real-time feedback control system in order to control film etching and growth rate. The main shortcoming of this method is: the process gas easily affects the vacuum windows, causing them to be corroded or deposited so that the accuracy and reliability of the optical measurement is reduced.
Apart from this, Rashap et al. (IEEE Trans. Semiconduct. Manufact., Vol. 10, No. 1, pp. 137-146, 1995) used an optical emission spectroscopy, or, Knight et al. (IEEE Trans. Semiconduct. Manufact., Vol. 10, No. 1, pp.137-146, 1997) used a quadrupole mass spectrometry as sensor that coordinates with a driving device to form a real-time feedback control system in order to control the proportion of certain chemical species in the plasma source, and by controlling the proportion, to control etching rate and deposition rate. The shortcoming of this method is that it requires a relatively longer time to measure the plasma parameters, and consequently, the sampling frequency of the real-time feedback control system will become slower. This will affect the stability of the whole system. The object of the present invention is to provide a process and its system for fabricating plasma with feedback control on plasma density. This process uses a heterodyne millimeter wave interferometer as sensor to measure the plasma density in the process container and the plasma density that is needed in the plasma fabricating process, and then provides real-time information of the measurements to a digital control device which makes numerical calculations and then drives the RF power generator to change RF output power, so as to make dynamic correction on the abnormal situation of plasma source in the process container caused by the drifting of the driving device and external disturbance. In this way, the plasma density in the plasma process will be close to the expected plasma density. This ensures that each wafer in the plasma process is situated in a plasma source of similar density.
The present invention provides a process and its system for fabricating plasma with feedback control on plasma density. The goal of the process is to manufacture stable products through controlling plasma density in the plasma fabricating process. The process is comprised of the following steps: conducting numerical calculations through a controlling device according to the expected plasma density in the process and the measured real plasma density; and regulating the output power of an RF power generator so as to maintain the plasma density close to the expected value.